1. Field of the Invention
This invention relates to a method for the detection of plant pathogens in seeds and, more particularly, to a method of detecting seed-transmitted plant pathogens.
2. Description of the Related Art
Plant pathogens spread in the manner of aerial infection, contact transmission or rainwater transmission, and the like, and cause great damage to agricultural productions. Among them, seed-transmissible pathogens, which are serious as primary sources of infection of diseases, do a lot of damage even through a seeds-lot with a low degree of contamination, and the damage is sometimes especially serious in transplant production facilities where a large number of seedlings are raised or in graft cultivation. These seed-transmissible plant pathogens cause various diseases, including bacterial fruit blotch of cucurbit and bacterial brown stripe of rice, and the damage caused thereby is not slight.
The main sources of infection causing such seed-transmitted diseases are seeds contaminated with the corresponding pathogens. A conventional method of detecting contaminated seeds is the grow-out test or sweatbox grow-out test which comprises sowing seeds in compost in a greenhouse or in a plastic box and, after germination, evaluating the seedlings by the occurrence or nonoccurrence of some disease symptom or other. Also known are the methods comprising immersing seeds in a solution and stirring and/or pressing the mixture for washing seeds, using the seed washings solution as such or preparing such a solution by a certain period of incubation at a certain temperature, and using the solution as a sample and subjecting the same to identification of colonies grown on medium, serological detection, genetic diagnosis, or a combination of some of these (cf. e.g. Non-Patent Document: Agriculture, Forestry and Fisheries Research Council Secretariat (December 2002): Development of methods to control bacterial fruit blotch in watermelon, Agriculture, Forestry and Fisheries Research Council, Tokyo).
In carrying out the grow-out test mentioned above, it is necessary to appropriately control the greenhouse conditions to maintain the temperature and humidity in the greenhouse at certain respective levels suited for the growth of seed-transmissible pathogens and the symptom development. The sweatbox grow-out test, which is an improved version of the grow-out test, is carried out in an artificial environment, so that the environment is always under adequate control; since, however, it is necessary to test at least 10000 or more seeds, the facilities required for inspection constitute a heavy burden from the equipment and cost viewpoint.
The percent germination of the seeds to be inspected is not always 100% and, in case of the only or all the contaminated seeds failing to germinate, there arises the problem that the seeds of the relevant lot are erroneously evaluated as healthy and safe. Furthermore, each seed-transmissible pathogen species generally includes several strains, either stronger or weaker in pathogenicity in a crop species; therefore, there is another problem that if there is a contaminated seed but the seedling therefrom is symptomless, the seeds of the relevant lot are evaluated as healthy and safe. Thus, it has been earnestly demanded that a highly precise seed health testing method be developed.